The invention relates generally to power supplies for electronic devices, and more particularly to soft-switching power supplies.
Regulated power supplies are found in virtually all electronic devices, including battery chargers, cellular telephones, computers, computer monitors, televisions, audio equipment, and video cameras. One typical power supply, the DC to DC converter, operates from a DC source, generates an alternating current as an intermediate process, and delivers direct current to a load. Switched DC converters, like linear, transformer-based power supplies, deliver regulated output and provide isolation between input and output circuits. Unlike a linear power supply, however, the transformers in a switched DC converter operate at much higher frequencies, as high as several megahertz. This permits the use of small components, including transformers and capacitors, while still providing for complete isolation between the input and the output.
Despite the advantages of switched DC converters, they are known to introduce radiated losses during switching. In hard-switching topologies, for example, switching causes a rapid transition in the current through the switch. This rapid transition causes switching losses in the form of signal emissions. These switching transients have a spectrum containing high frequency components, which can introduce noise in video signals or the like. The soft-switching switching converter is a known converter topology that reduces hard switching losses. In a typical soft-switcher, a resonator forces the current in the power switches to zero during switching. This significantly reduces switching transients and the radiated emissions caused by rapid transitions in the switching current.
Soft-switchers, however, also fail to achieve optimum efficiency. Leakage inductance between the primary and auxiliary windings of the transformers commonly used in soft-switchers introduces power loss; the uncoupled magnetic flux causes voltage peaks during the current changes induced by switching. These switching transients also contain high frequency components that appear as radiated emissions. Furthermore, in known soft-switching topologies, the interwinding capacitance between the primary and auxiliary windings appears in the resonator circuit loop. This capacitance introduces secondary oscillations and concomitant power losses. Due to the xe2x80x9cproximity effect,xe2x80x9d soft-switchers also experience significant eddy current losses. The time varying current in the primary winding causes a non-uniform current distribution over the cross section of the conductors in the auxiliary winding.
A soft-switching power supply according to the principles of the invention reduces radiated emissions and the secondary oscillations caused by interwinding capacitance in the transformer. The resonator is coupled between one leg of the transformer auxiliary winding and an auxiliary switch network. The primary winding is interposed between a voltage and a primary switch network. The primary switch network and the auxiliary switch network control the application of energy to the primary and auxiliary windings. The resonator forces the current in the switch to the desired level during switching. Regulation is achieved by feeding back the output voltage (voltage mode) or the current in the primary switch (current mode) to a controller that provides the switch control signals for the primary and auxiliary switches. By placing the resonator in series with the auxiliary switch network, the effect of the interwinding capacitance is reduced; the interwinding capacitance couples to ground through the other leg of the auxiliary winding.
This topology permits the use of transformers with closer coupling than has been previously used in soft-switching topologies. Two such transformers are the multifilar or bifilar transformer. In a bifilar transformer, the primary windings and the auxiliary windings are made together and interleaved so that adjacent wires always belong to a different winding. In multifilar windings, each winding contains multiple strands of interleaved wires. These winding techniques yield closer coupling, minimize leakage inductance and reduce eddy current loss due to the proximity effect.